As quartz glass based material is used in the core layer of conventional optical waveguides, physical properties of the core layer such as the refractive index and coefficient of thermal expansion are roughly determined by the physical properties of quartz glass. In recent years, there is sometimes a request for changing the physical properties of the core layer. For this reason, proposals have been made to form optical waveguides by using multi-component glass containing such elements as alkaline elements, alkaline earth elements, or rare earth elements. When forming a core layer of an optical waveguide with multi-component glass, physical properties of the glass such as refractive index, thermal expansion coefficient, or glass transition temperature can be freely designed over a wider range by properly changing the glass composition. As a practical method, a core layer is formed on a first clad layer by using glass containing alkaline elements, then the core layer is processed by dry-etching, followed by formation of a second clad.
A problem encountered with the core layer formed with the above-mentioned multi-component glass is that the optical transmission loss of the optical waveguide tends to be large. To be more specific, when making a core pattern from a core layer formed with a conventional multi-component glass, dry etching is performed with a mask or a resist pattern provided over the core layer. During this process, large irregularities are formed on the periphery of the core pattern that has been formed, and the irregularities cause an increase in the optical transmission loss of the optical waveguide.